The present invention relates to a method for selective etching of a sub-area of a substrate during the fabrication of a semiconductor object. More particularly, it relates to the etching of an oxide layer on a silicon-based semiconductor wafer using vapors from a reagent that is the source for a reactive species of fluorine. It particularly relates to use of hydrogen fluoride (HF) vapors. The invention further relates to an apparatus for selectively etching a selected area of a substrate. More particularly, the invention relates to an apparatus and method for etching material from alignment markers on a silicon wafer.
The fabrication of integrated circuits on silicon wafers uses many differing processes and materials. For instance, photolithographic techniques are used to pattern the various gates on the silicon chip. As sophisticated pattern definition technologies have been developed, the geometry of integrated circuit components has shrunk from the six-micron size of the 1970s, to the sub-micron technologies of the late 1980's, to the deep sub-micron regions of the 1990s. Therefore, it has become increasingly important to carefully align the wafer during semiconductor device manufacturing processes. Also, as the size of the features of the integrated circuits have become increasingly smaller and the spacing of the features of the semiconductor devices has decreased on the wafer, of necessity, the size of any predetermined area of the wafer containing any feature or circuit component on the wafer has decreased. This is the case for the alignment markers on the wafer used to align the wafer during manufacturing processes.
The alignment marks can easily become covered or contaminated with various materials during processing steps. For example, after photoresist material is applied to the wafer in a circuit forming process and the subsequent etching of the wafer to form the desired circuit, the wafer's alignment markers may remain covered with photoresist material or subsequently deposited oxide material. Such material must be removed prior to the continued processing of the wafer. If it is not, optical wafer alignment systems may not find the alignment markers. Therefore, alignment markers on the wafer must be cleaned before any subsequent processing occurs to ensure proper alignment of the wafer on the process equipment.
It is difficult to remove such material from the alignment markers because the alignment markers have a pattern forming low trenches where the undesired material resides. While mechanical polishing removes photoresist and oxide materials from the plane surface of the wafer, the material in the trenches is not easily removed by polishing.
Therefore, it is desirable to have a method and apparatus for cleaning predetermined areas of the wafer, such as the alignment marks on a wafer.
In U.S. Pat. No. 5,271,798, owned by the assignee of the present invention, and hereby incorporated by reference as if set forth herein in its entirety, method and apparatus are illustrated for the cleaning of alignment markers on a wafer. The apparatus uses a cylindrical containment nozzle that engages the area surrounding an alignment marker on a wafer. When the nozzle is sealed against the wafer, an etchant is dispensed over the alignment marker. The etchant is removed from the alignment mark by suction applied though the nozzle. This prior art method and apparatus requires the nozzle to physically contact the wafer to form a seal around the alignment mark. A similar concept is disclosed in Ser. No. 08/916,997 entitled "Method and Apparatus for Selective Removal Of Material From Wafer Alignment Marks", filed Aug. 20, 1997, which is commonly owned by the assignee of the present invention, and which is hereby incorporated by reference as if set forth herein in its entirety.
That patent application also discloses a process and apparatus for removing material from a predetermined area, such as an alignment marker. The etching apparatus uses a nozzle for dispensing etchant onto the alignment marker. The nozzle can form a "virtual seal" with the wafer that is based on close but not actual physical contact with the wafer. The virtual seal is intended to overcome any concern that the nozzle may damage the wafer through physical contact. The virtual seal works by creating a lower pressure in the nozzle than the pressure of the surrounding atmosphere so that the nozzle draws in atmospheric air. By maintaining a lower pressure in the nozzle, etchant dispensed through the nozzle does not escape the nozzle.
In both the aforementioned patent application and the '798 patent, the containment nozzle has a tubular member disposed concentrically within an outer annular member for surrounding an alignment marker. Etchant is dispensed through the tubular member onto the wafer. Etchant is removed through an annular space between the tubular member and the outer annular member by suction.
In practice, the aforementioned methods and systems have been used with liquid etchants. The liquid etchants are delivered under positive pressure onto the wafer surface. The etchant is then removed under a relatively high vacuum to keep it from escaping the nozzle containment area. One significant is problem with liquid etchants delivered through prior art apparatus is that the liquid etchants are transported outside the containment nozzle because of surface tension forces. This is true even for virtual-seal nozzles applying a negative pressure. The liquid etchant spilling outside the nozzle containment area can damage features and circuits on the wafer. Therefore, there is a need for improved liquid dispensation systems that minimize or prevent etchants from spilling outside the nozzle.
Another problem with existing methods and systems arises from the fact that they use a positive pressure to dispense a liquid etchant onto the wafer. Because of the use of positive pressure, if the nozzle's seal fails, liquid will continue to be dispensed under pressure spilling or spraying outside the nozzle onto the wafer and surrounding equipment. Such spillage may not only damage the wafer but it also poses a safety risk to equipment that may be exposed to highly corrosive and toxic etchants.
A related disadvantage of prior art systems is that such systems do not inherently shut down if a seal fails. Therefore, if a seal does fail, the positive pressure will continue driving the etchant from the containment nozzle until there is a shutdown. Accordingly, added features must be incorporated into the system to shut it down in case of seal failure.
A further problem in existing system arises from the concentric arrangement of the containment nozzle described above. This arrangement does not allow optimal flow of etchant on the selected surface on the wafer or optimal contact time with the surface. This can lead to irregularities in the etched surface due to incomplete or over-etching.
In view of the foregoing problems and disadvantages, there is a need for improved methods and systems for processing selected areas of a wafer. In particular, there is a need for improved methods and systems directed at etching alignment markers of any material, including photoresist materials, chemical-mechanical-planarization process materials, and refractory metals.